In recent years, a radiation detector, comprising a phosphor film to convert radiation, particularly X-rays into light, and a photoelectric conversion element to convert the light into an electric signal, has been practically used. Such a radiation detector can reduce the size and weight of radiological equipment. The radiation detector converts image data obtained by radiation passing through an object, into digital electronic data. The radiation detector has great convenience in digital data processing such as digital image processing and digital image storage.
The radiation detector has been used in a wide field including medical and dental uses which are used for diagnosis and treatment for patients, industrial use of nondestructive inspection, scientific inquiry of structural analysis, etc. Digital data processing enables precision image extraction and high-speed image detection in each field. The degree of undesired exposure to radiation can be reduced, and speedy inspection and diagnosis can be realized.
Scintillator technology is often used for a phosphor film of a radiation detector. A scintillator is made of material consisting mainly of Cs and I used for a conventional X-ray image tube. The scintillator material consisting mainly of cesium iodine (CsI) and forming a columnar crystal can improve sensitivity and resolution by an optical guide effect, compared with other scintillator materials forming a particulate crystal.
A conventional X-ray image tube needs an electronic lens in a vacuum tube, and increases the size and weight. However, a thin two-dimensional radiation detector is possible by making a photodetector having a photoelectric conversion element of a thin-film element using amorphous silicon.
A photodetector and a circuit board, for example, are arranged in parallel to each other so as to realize an advanced thin lightweight radiation detector. A phosphor film is used in the photodetector. The circuit board electrically drives the photodetector, and electrically processes an output signal from the photodetector.
A flexible circuit board connects the photodetector and circuit board. The flexible circuit board is provided with a shift register, and an integrated-circuit (IC) semiconductor element for detecting a signal. The above technique of reducing the whole size of a radiation detector is disclosed in Jpn. Pat. Appln. KOKAI Publication No. H8-116044 (page 33, FIG. 52).
On the other hand, the radiation detector has been improved in the performance. Particularly, a power supply noise is reduced to improve the output signal stability and signal-to-noise ratio. For example, a low-pass filter is connected to each reference power supply. Such a technique intends to reduce a random noise in each reference voltage, and to prevent fluctuations and degradation of signal-to-noise ratio in an output signal caused by noise in such reference voltages. The technique is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-163343 (pages 3-4, FIG. 1).
The above described conventional technology causes the following problems.
The flexible circuit board is highly flexible, and effective for electrical connection between parallel-connected photodetector and circuit board. However, a local stripping stress may occur in the electrical connection part of the signal detection IC semiconductor element mounted on the flexible circuit board. Therefore, the flexible circuit board may raise a problem such as stripping, which degrades the reliability of the electrical connection part.
Further, as the flexible circuit board is highly flexible, the signal detection IC semiconductor element is likely to move when an external force is applied to the radiation detector. The larger the mass of the signal detection IC semiconductor element, the vibration or shock is greater. When a great vibration or shock occurs, the electrical connection part is mechanically damaged.
Further, when a vibration occurs in the radiation detector, the flexible circuit board is easy to be deformed. The deformed flexible circuit board causes a fluctuation in the floating capacitance of the electrical traces on the flexible circuit board. As a result, noise occurs in the reference power supply voltage, input signal, and output signal of the signal detection IC semiconductor element mounted on the flexible circuit board. Therefore, the radiation detector cannot display full performance even if a noise protective measure is taken for the circuit.
The flexible circuit board provided with a signal detection IC semiconductor element is contracted in the form of a tape carrier package (TCP) or chip on film (COF). Thus, other electronic parts such as a capacitor cannot be arranged or mounted close to the signal detection IC semiconductor element on the flexible circuit board. These electronic parts are used for generating a reference potential necessary for the signal detection IC semiconductor element. This increases a noise in the voltage or signal.
A multilayer structure is problematic with respect to the traces for a power supply, grounding, a reference potential, a control signal, and a detection signal. These traces are arranged in parallel on a flat surface. The trace for a detection signal is susceptible to noise, and is likely to permit noise.
Impedance is high in the traces for a power supply and grounding in the signal detection IC semiconductor element. Thus, the power supply voltage is decreased in an input terminal part of the signal detection IC semiconductor element. This may affect an offset of grounding, and fixed noise.